NCERT Exemplar Solutions for Class 12 Biology chapter 1 Reproduction in Organisms

These Solutions are part of NCERT Exemplar Solutions for Class 12 Biology. Here we have given NCERT Exemplar Solutions for Class 12 Biology chapter 1 Reproduction in Organisms

Multiple Choice Questions


Question 1.
A few statements describing certain features of reproduction are given below.
(i) Gametic fusion takes place.
(ii) Transfer of genetic material takes place.
(iii) Reduction division takes place.
(iv) Progeny have some resemblance with parents.
Select the options that are true for both asexual and sexual reproduction from the options given below.
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (ii) and (iv)
(d) (i) and (iii)
Answer:
(c) : Reproduction is a biological process in which-an organism produces young ones (offspring) similar to itself. In reproduction, offsprings have some resemblance with parents. Both sexual and asexual reproduction involve transfer of genetic material.

Question 2.
The term ‘clone’ cannot be applied to offspring formed by sexual reproduction because
(a) offspring do not possess exact copies of parental DNA
(b) DNA of only one parent is copied and passed on to the offspring
(c) offspring are formed at different times
(d) DNA of parent and offspring are completely different.
Answer:
(a) : In sexual reproduction, there is fusion of male gametes and female gametes, the offspring produced are not identical to their parents. This genetic recombination leads to variations, which play an important role in evolution.

Question 3.
A sexual method of reproduction by binary fission is common to which of the following?
(i) Some eukaryotes
(ii) All eukaryotes
(iii) Some prokaryotes
(iv) All prokaryotes
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (i) and (iii)
(d) (iii) and (iv)
Answer:
(c)

Question 4.
A few statements with regard to sexual reproduction are given below.
(i) Sexual reproduction does not always require two individuals.
(ii) Sexual reproduction generally involves gametic fusion.
(iii) Meiosis never occurs during sexual reproduction.
(iv) External fertilisation is a rule during sexual reproduction.
Choose the correct statements from the options below.
(a) (i) and (iii)
(b) (i) and (ii)
(c) (ii) and (iii)
(d) (i) and (iv)
Answer:
(b) : Meiosis is required for the production of haploid gametes during sexual reproduction. External fertilisation is not a rule during sexual reproduction, it can occur internally also.

Question 5.
A multicellular, filamentous alga exhibits a type of sexual life cycle in which the meiotic division occurs after the formation of zygote. The adult filament of this alga has
(a) haploid vegetative cells  and   diploid gametangia
(b) diploid vegetative cells  and   diploid gametangia
(c) diploid vegetative cells  and  haploid gametangia
(d) haploid vegetative cells and  haploid gametangia.
Answer:
(d)

Question 6.
The male gametes of rice plant have 12 chromosomes in their nucleus. The chromo­some number in the female gamete, zygote and the cells of the seedling will be, respectively
(a) 12,24,12
(b) 24,12,12
(c) 12,24,24  
(d) 24,12,24.
Answer:
(c) : Chromosome number in male gamete of rice plant is n = 12 therefore chromosome number in female gamete would also be 12. Zygote is diploid as it is the product of fertilisation and the cells of the seedling would be meiocytes and other diploid cells. Hence, the chromosome number in both zygote and cells of seedling will be 2n = 24.

Question 7.
Given below are a few statements related to external fertilisation. Choose the correct statements.
(i) The male and female gametes are formed and released simultaneously.
(ii) Only a few gametes are released into the medium.
(iii) Water is the medium in a majority of organisms exhibiting external fertilisa­tion.
(iv) Offspring formed as a result of external fertilisation have better chance of survival than those formed inside an organism.
(a) (iii) and (iv)
(b) (i) and (iii)
(c) (ii) and (iv)
(d) (i) and (iv)
Answer:
(b) : A large number of gametes are released into the medium to increase the chances of fertilisation. The chances of survival of offsprings from external fertilisation are lesser than those of internal fertilisation as they face more risk from predators.

Question 8.
The statements given below describe certain features that are observed in the pistil of flowers.
(i) Pistil may have many carpels.
(ii) Each carpel may have more than one ovule.
(iii) Each carpel has only one ovule.
(iv) Pistil have only one carpel.
Choose the statements that are true from the options below.
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iv)
(d) (iii) and (iv)
Answer:
(a) : Gynoecium is the female part of the flower, a unit of which is called pistil. A pistil may have one or more than one carpels (monocarpellary, bicarpellary etc). Each carpel may have more than one ovules.

Question 9.
Which of the following situations correctly describe the similarity between an angiosperm egg and a human egg?
(i) Eggs of both are formed only once in a lifetime.
(ii) Both the angiosperm egg and human egg are stationary.
(iii) Both the angiosperm egg and human egg are motile transported.
(iv) Syngamy in both results in the formation of zygote.
Choose the correct answer from the options given below.
(a) (ii) and (iv)
(b) (iv) only
(c) (iii) and (iv)
(d) (i) and (iv)
Answer:
(b) : Syngamy is the complete and permanent fusion of male and female gametes to form a zygote.

Question 10.
Appearance of vegetative propagules from the nodes of plants such as sugarcane and ginger is mainly because
(a) godes are shorter than internodes
(b) nodes have meristematic cells
(c) nodes are located near the soil
(d) nodes have non-photosynthetic cells.
Answer:
(b) : Vegetative propagation is asexual reproduction from ‘various parts in plants. In plants, such as, sugarcane and ginger, appearance of vegetative propagules from nodes is because of presence of meristematic cells in them. Meristematic cells have the ability to divide to form new cells which can differentiate and give rise to permanent tissues.

Question 11.
Which of the following statements, support the view that elaborate sexual reproductive process appeared much later in the organic evolution?
(i) Lower groups of organisms have simpler body design.
(ii) Asexual reproduction is common in lower groups.
(iii) Asexual reproduction is common in higher groups of organisms.
(iv) The high incidence of sexual reproduction in angiosperms and vertebrates.
Choose the correct answer from the options given below.
(a) (i), (ii) and (iii)
(b) (i), (iii) and (iv)
(c) (i), (ii) and (iv) 
(d) (ii), (iii) and (iv)
Answer:
(c)

Question 12.
Offspring formed by sexual reproduction exhibit more variation than those formed by asexual reproduction because
(a) sexual reproduction is a lengthy process
(b) gametes of parents have qualitatively different genetic composition
(c) genetic material comes from parents of two different species.
(d) greater amount of DNA is involved in sexual reproduction.
Answer:
(b) : Sexual reproduction occurs in almost all types of animals and mostly in higher plants. It is usually biparental. Daughter organisms genetically differ from the parents. Since there are variations, so it contributes to evolution of the species.

Question 13.
Choose the correct statement from amongst the following.
(a) Dioecious organisms are seen only in animals.
(b) Dioecious organisms are seen only in plants.
(c) Dioecious organisms are seen in both plants and animals.
(d) Dioecious organisms are seen only in vertebrates.
Answer:
(c) : Dioecious organisms are those in which male and female sex organs are present in different organisms, where as monoecious organisms are those in which male and female sex organs are present in the same organism. Monoecious organisms are also called as hermaphrodite. Dioecious organisms are seen in both plants and animals. Papaya, date palm and most of the animals are dioecious.

Question 14.
There is no natural death in single celled organisms like Amoeba and bacteria because
(a) they cannot reproduce sexually
(b) they reproduce by binary fission
(c) parental body is distributed among the offspring
(d) they are microscopic.
Answer:
(c) : These are no natural death in single celled organisms like Amoeba and bacteria. It is so, because of asexual reproduction, the body of parent is divided into daughter cells. So, in effect, there is no practical death in Amoeba and bacteria.

Question 15.
There are various types of reproduction. The type of reproduction adopted by an organism depends on
(a) the habitat and morphology of the organism
(b) morphology of the organism
(c) morphology and physiology of the organism
(d) the organism’s habitat, physiology and genetic makeup.
Answer:
(d) : There are various types of reproduction, both asexual (fission, budding, etc.) and sexual (internal and external). The type of reproduction, an organism undergoes depends ultimately on its genetic makeup which influences its physiology. Habitat also influences the type of reproduction, that organism undergoes.

Question 16.
Identify the incorrect statement.
(a) In asexual reproduction, the offspring produced are morphologically and genetically identical to the parent.
(b) Zoospores are sexual reproductive structures.
(c) In asexual reproduction, a single parent produces offspring with or without the formation of gametes.
(d) Conidia are asexual structures in
Answer:
(b) : Spores formation is also a type of asexual reproduction. Zoospores, conidia, oidia, etc. are all asexually reproducing structures. There is generally no gamete formation in asexual reproduction and the offsprings produced are called clones.

Question 17.
Which of the following is a post-fertilisation event in flowering plants?
(a) Transfer of pollen grains
(b) Embryo development
(c) Formation of flower
(d) Formation of pollen grains
Answer:
(b) : Events in sexual reproduction after the fertilisation are called post-fertilisation events. After fertilisation, a diploid zygote is formed in all sexually reproducing organisms. The process of development of embryo from the zygote is called embryogenesis.

Question 18.
The number of chromosomes in the shoot tip cells of a maize plant is 20. The number of chromosomes in the microspore mother cells of the same plant shall be
(a) 20 
(b) 10
(c) 40
(d) 15.
Answer:
(a) : Shoot tip cells and microspore mother cells both are diploid in maize plant. If number of chromosomes in shoot tip cell (2n) = 20, then number of chromosomes in microspore mother cell will be (2n) = 20.

Very Short Answer Type Questions

Question 1.
Mention two inherent characteristics of Amoeba and yeast that enable them to reproduce asexually.
Answer:
The two inherent characteristics for asexual reproduction in Amoeba and yeast are :

  1. Gametes are not formed in Amoeba and yeast.
  2. Uniparental condition (i.e., single parent) is involved in reproduction of both Amoeba and yeast.

Question 2.
Why do we refer to offspring formed by asexual method of reproduction as clones?
Answer:
Offsprings produced by asexual reproduction are morphologically and genetically similar to their parents. Hence, they are known as clones.

Question 3.
Although potato tuber is an underground part, it is considered as a stem. Give two reasons.
Answer:
Reasons for considering potato as a stem are:

  1. Differentiation into nodes and inter­nodes.
  2. The nodes bear buds which can grow to from leaf shoots or plantlets.

Question 4.
Between an annual and a perennial plant, which one has a shorter juvenile phase? Give one reason.
Answer:
Annual plant have shorter juvenile phase as they complete their life cycle in single season i.e., a few weeks to a few months, e.g., wheat, maize, pea, gram.

Question 5.
Rearrange the following events of sexual reproduction in the sequence in which they occur in a flowering plant : embryogenesis, fertilisation, gametogenesis, pollination.
Answer:
The correct sequence of events of sexual reproduction in flowering plants are as follows : gametogenesis, pollination, fertilisation, embryogenesis.

Question 6.
The probability of fruit set in a self-pollinated bisexual flower of a plant is far greater than a dioecious plant. Explain.
Answer:
Presence of male and female repro­ductive organs on same plant provides more chances of self pollination and therefore increases chances of fruit set. However in dioecious plants agents of pollination are needed which may therefore decrease the chances of a fruit set.

Question 7.
Is the presence of large number of chromosomes in an organism a hindrance to sexual reproduction? Justify your answer by giving suitable reasons.
Answer:
The presence of a large number of chromosomes in an organism is not a hindrance to sexual reproduction.

Question 8.
Is there a relationship between the size of an organism and its life span? Give two examples in support of your answer.
Answer:
There is no direct correlation between the life span of organisms and their sizes.

  1. The crow and parrot are nearly of the same size, but life span of crow and parrot are 15 years and 140 years respectively.
  2. The mango and the peepal trees are nearly of the same size but the life span of mango and peepal are 100 years and 1000 years respectively.

Question 9.
In the figure given below the plant bears two different types of flowers marked ‘A’ and ‘B’. Identify the types of flowers and state the type of pollination that will occur in them.
NCERT Exemplar Solutions for Class 12 Biology chapter 1 Reproduction in Organisms 1

Answer:
The plant shown in figure has two types of flowers:
Flower A – chasmogamous type with stigma and anthers exposed. Such type of flowers may undergo self or cross pollination.
Flower B – Cleistogamous type – closed type of flowers, where stigma and anthers are not exposed. Such type of flowers undergo only self pollination.

Question 10.
Give reasons as to why cell division cannot be a type of reproduction in multicellular organisms.
Answer:
In multicellular organisms cell division does not divide the whole body into daughter cells as in unicellular organisms. But multicellular organisms have a well developed reproductive organs which help in reproduction.

Question 11.
In the figure given below, mark the ovule and pericarp.
NCERT Exemplar Solutions for Class 12 Biology chapter 1 Reproduction in Organisms 2
Answer:
NCERT Exemplar Solutions for Class 12 Biology chapter 1 Reproduction in Organisms 3

Question 12.
Why do gametes produced in large numbers in organisms exhibit external fertilisation?
Answer:
In external fertilisation, there are great chances that the sperm and the eggs released by the organisms can be affected by desiccation, predators, etc. So, to make up for the high fatality rate of the gametes, the organisms produces a lot of gametes.

Question 13.
Which of the followings are monoecious and dioecious organisms.
(a) Earthworm____________
(b) Chara ______________
(c) Marchantia____________
(d) Cockroach ___________
Answer:
(a) Earthworm – Monoecious animal
(b) Chara – Monoecious plant
(c) Marchantia- Dioecious plant
(d) Cockroach – Dioecious plant

Question 14.
Match the organisms given in Column ‘A’ with the vegetative propagules given in Column ‘B’

Column A Column B
i Bryophyllum a. Offset
ii Agave b. Eyes
iii. Potato c. Leaf buds
iv. Water hyacinth d. Bulbils
  1. Rrunphyllum. – (c) Leaf buds
  2. Agave –  (d)   Bulbils
  3. Potato –  (b)   Eyes
  4. Water hyacinth –  (a) Offset

Question 15.
What do the following parts of a flower develop into after fertilisation?
(a) Ovary__________
(b) Ovules__________
Answer:
(a) Ovary – fruit
(b) Ovules-seed

Short Answer Type Questions

Question 1.
In haploid organisms that undergo sexual reproduction, name the stage in the life cycle when meiosis Give reasons for your answer.
Answer:
In zygospore (formed by zygote) meiosis occurs. Because after meiosis it can form meiospores which can develop into haloid organisms.

Question 2.
The number of taxa exhibiting asexual reproduction is drastically reduced in higher plants (angiosperms) and higher animals (vertebrates) as compared with lower groups of plants and animals. Analyse the possible reasons for this situation.
Answer:
Both angiosperms and vertebrates have more complex structural organisation. They have evolved very efficient mechanism of sexual reproduction.

  1. The offspring produced due to sexual reproduction adapt better to the changing environmental conditions.
  2. Genetic recombination, interaction, etc. during sexual reproduction provide vigour and vitality to the offspring.

Question 3.
Honeybees produce their young ones only by sexual reproduction. Inspite of this, in a colony of bees we find both haploid and diploid  individuals Name the haploid and diploid individuals in the colony and analyse the reasons behind their formation.
Answer:
Honeybees produce their young ones by sexual reproduction but parthenogenesis also occurs alongwith sexual reproduction. Fertilised eggs and parthenogenetically developed eggs give rise to different castes. In honey bees, fertilised eggs (zygotes) which is diploid give rise to queens and workers (both are females) and unfertilised eggs (ova) which is haploid develop into drones (males).

Question 4.
With which type of reproduction do we associate the reduction division? Analyse the reasons for it.
Answer:
We can associate the reduction division (meiosis) with sexual reproduction. Sexual reproduction is the process of development of new organisms through the formation and fusion of male and female gametes. It involves meiosis or reduction division i.e., halving the parental chromosomes inside the male and female gamete and their subsequent fusion resulting in the restoration of the original parental number of chromosomes, causing an increased genetic diversity.

Question 5.
Is it possibletoconsider vegetative propagation observed in certain plants like Bryophyllum, water hyacinth, ginger etc., as a type of asexual reproduction? Give two/ three reasons.
Answer:
Yes, it is possible to consider vegetative propagation as a type of asexual reproduction because of the following reasons:

  1. Production of plantlet occurs by a single parent plant without the formation and fusion of gametes.
  2. Plantlets receives all genes from their one parent plant.
  3. Vegetative reproduction involves only mitotic cell division.
  4. There is no gamete formation.

Question 6.
Fertilisation is not an obligatory event for fruit production in certains plants’ Explain the statement.
Answer:
No, fertilisation is not an obligatory event for fruit production in certain plants, as fruits can also develop from unfertilised ovary also. Such fruits are called parthenocarpic  fruits. The formation of this type of fruit takes place without prior’fertilisation of the flower by pollen. The resulting fruits are seedless and therefore do not contribute to the reproduction of the plant, examples are bananas and pineapples. Plant growth substances may have a role in this phenomenon, which can be induced by auxins in the commercial production of tomatoes and other fruits.

Question 7.
In a developing embryo, analyse the consequences if cell divisions are not followed by cell differentiation.
Answer:
Cell division increase the number of cells in the developing embryo while cell differentiation helps the groups of cells to undergo certain modifications and form specialised tissues and organs.

Initially all the cells of the developing embryo are alike with same genetic information. Later however, due to the phenomenon of differentiation depending on their location and internal cellular mechanism, different cells of the embryo develop differently forming embryonal axis, plumule and radicle. However, if cell differentiation does not occur the embryo will remain just a mass of undifferentiated cells or callus. There would not any plumule, radicle, cotyledons or embryo axis. A new plant will not be formed from such an embryo.

Question 8.
List the changes observed in an angiosperm flower subsequent to pollination and fertilisation.
Answer:
In an angiosperm flower, the changes that occur subsequent to pollination and fertilisation are called post-fertilisation changes. Pollen grain germinates over the stigma and forms a pollen tube carrying two gametes. Pollen tube reaches ovary and enters an ovule through one of its synergids. Fertilisation produces a diploid zygote and triploid primary endosperm cell. Zygote produces embryo and primary endosperm cell forms endosperm. Transformation takes place and ovule transforms into seed, ovary transforms into fruit and ovary wall into pericarp. The petals, stamens and style wither away.

Question 9.
Suggest a possible explanation why the seeds in a pea pod are arranged in a row, whereas those in tomato are scattered in the juicy pulp.
Answer:
Arrangement of seeds inside a fruit depends upon the type of placentation and the growth of placental axis. In pea ovary, the ovules are attached to the ventral suture i.e., marginal placentation and carpel is mono- carpellary. So, seeds are arranged in a row. In tomatoes, the gynoecium is tricarpellary with axile placentation. The placentae grow and become pulpy during fruit formation. As a result seeds get scattered in the pulpy mass in tomato.

Question 10.
Draw the sketches of a zoospore and a conidium. Mention two dissimilarties between them and atleast one feature common to both structures.
Answer:
NCERT Exemplar Solutions for Class 12 Biology chapter 1 Reproduction in Organisms 4
Dissimilarities : Zoospores are motile and flagellated whereas conidia are nonmotile. Zoospore are formed inside a zoosporangium and conidia are formed (exogenously) outside on a conidiophore.
Similarity : Both are reproductive structures for asexual reproduction.

Question 11.
Justify the statement’Vegetative reproduction is also a type of asexual reproduction’.
Answer:
Refer answer 5.

Long Answer Type Questions

Question 1.
Enumerate the differences between asexual and sexual reproduction. Describe the types of asexual reproduction exhibited by unicellular organisms.
Answer:

A sexual reproduction Sexual reproduction
(1) Asexual reproduction involves the participation of single individual parent. Sexual reproduction involves participation of two separate parents.
(2) It generally occurs without the involvement of sex organs. It usually involves the sex organs.
(3) It does not involve meiosis or reduction division. It involves meiosis which occurs at the time of gamete formation.
(4) Asexual reproduction does not involve sexual fusion or fusion of two gametes. Zygotes are not formed. The sexual reproduction requires fertilisation to take place between two opposite gametes leading to the production of a zygote.
(5) Since asexual reproduction does not involve meiosis and fusion of gametes, the offsprings are genetically similar to parents and they do not show variation. The individuals produced as a result of meiosis and gametic fusion exhibit genetic variation and differ from either of the two parents.
(6) It is a very quick method of multiplication and, therefore, used by plant breeders for cloning. It is very slow method of multiplication of individuals.

Asexual reproduction occurs usually in some organisms such as monerans, protists, in plants and in some animals.
Following are the methods of asexual reproduction in unicellular organisms :

(1) Binary fission : In this type of asexual reproduction, the parent organism divides into two halves, each half forms an independent daughter organism e.g.,

(2) Multiple fission : In this process, the parent body divides into many similar daughter individuals. Multiple fission occurs in Amoeba during unfavourable condition.

(3) Bidding : In budding, a daughter individual is formed from a small part or bud, arising from the parent body g., yeast.
Spore formation or sporulation : Spores are minute, single celled, thin or thick walled propagules. In this type of asexual reproduction dispersive structures called spores are released from parent body that germinate under favourable conditions to form new individuals. Motile spores called zoospores are formed in unicellular alga like

Question 2.
Do all the gametes formed from a parent organism have the same genetic composition (identical DNA copies of the parental genome)? Analyse the situation with the background of gametogenesis and provide or give suitable explanation.
Answer:
No, All the gametes formed from a parent organism don’t have the same genetic combination. Formation of two types of gametes-male and female, inside the gametangia, is called gametogenesis. The reproductive units in sexual reproduction are specialised cells called gametes. The gametes are generally of two kinds : male and female. The gametes of all the organisms are usually haploid cells, i.e., possess single set (or n number) of chromosomes. The gametes are usually formed by meiotic divisions. Therefore, they are haploid, i.e., have halved or reduced (n) number of chromosomes. During this processes, random segregation of chromosomes (independent assortment) and exchange of genetic material between homologous chromosomes (crossing over) result in new combinations of genes in the gametes, and this reshuffling increases genetic diversity. The coming together of two unique sets of chromosomes in the zygote forms the genetic basis of variation within the species.

Question 3.
Although sexual reproduction is a long drawn, energy-intensive complex form of reproduction, many groups of organisms in Kingdom Animalia and Plantae prefer this mode of reproduction. Give atleast three reasons for this.
Answer:
Reasons for preference of sexual reproduction in higher groups of organisms are as follows:

  1. During sexual reproduction, there is fusion of gametes, hence genetic recombination takes place causing variations.
  2. The offspring produced due to sexual reproduction adapt better to the changing environmental conditions.
  3. Genetic recombination, interaction, etc. during sexual reproduction provide vigour and vitality to the offspring.
  4. Variation being a major factor of natural selection, therefore, it plays an important role in evolution.

Question 4.
Differentiate between
(a) oestrous and menstrual cycles;
(b) ovipary and vivipary. Cite an example for each type.
Answer:
(a) Differences between menstrual and oestrous cycles are as follows:

Menstrual cycle Oestrous cycle
(1) This cycle consists of menstrual phase, proliferative phase and the secretory phase. It consists of a short period of oestrous or heat (e.g., 12-24 hours in cow) followed by anoestrous or passive period
(2) Blood flows in the last few days of this cycle. Blood does not flow in this cycle.
(3) The broken endometrium is passed out during menstruation The broken endometrium is reabsorbed.
(4) Sex urge is not increased during menstruation. Sex urge is increased during oestrous period.
(5) Female does not permit copulation during menstrual phase of the cycle. Female permits copulation only during oestrous period.
(6) It occurs in primates (monkeys, apes and human beings) only. It occurs in nonprimates such as cows, dogs, etc

 (b) Differences between ovipary and vivipary are as follows:

Ovipary Vivipary
(1) The process in which females lay fertilised/ unfertilised eggs. The process in which females give birth to young ones.
(2) The development of zygote takes place outside the female’s body. The development of zygote takes place inside the female’s body.
(3) Females lay eggs in a safe place in the environment but the chances of survival are less. Females deliver young ones and the chances of survival are more.
(5) Example. All birds, most of reptiles and egg­laying mammals. Example. Mammals except egg-laying mammals

Question 5.
Rose plants produce large, attractive bisexual flowers but they seldom produce fruits. On the other hand a tomato plant produces plenty of fruits though they have small flowers. Analyse the reasons for failure of fruit formation in rose.
Answer:
Both rose and tomato plants are selected by human beings for different characteristics, the rose for its flower and tomato for its fruit. Rose plant is vegetatively propagated and does not produce seeds. Fruit formation fails in rose plant due to following reasons:

  1. Viable pollen may not be produced, resulting in failure of fertilisation.
  2. Functional egg may not be produced.
  3. The ovule produced may be defective or non functional.
  4. There may be pollen-pistil incom­patibility.
  5. The plant may be a hybrid with abnormal segregation of chromosomes resulting in non-viable gametes.
  6. The plant may be a hybrid and sterile.
  7. There may be internal barriers for growth of pollen tube and fertilisation

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